The government of Rio de Janeiro is building concrete walls to prevent sprawling slums from spreading farther into the picturesque hills of this world-famous tourist destination, an official said on Saturday. Construction has begun in two favelas, or shantytowns, in the southern districts of Rio de Janeiro, a government spokeswoman told Reuters. Officials say the wall is to protect the remaining native forest but critics fear the move could be seen as discriminatory and become a blemish symbolizing Brazil’s deep divisions between rich and poor.